Fix i939: Patmat shouldn't compare selectors to module TypeTrees.

It would be great to augment Ycheck to ensure that some trees are not
permitted, e.g. TypeTree cannot be a stat in a block or a receiver of
 a call.

Currently this is `ensured` by backend by failing.

Author: DarkDimius

src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -787,7 +787,8 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
         //  - Scala's arrays are invariant (so we don't drop type tests unsoundly)
         if (extractorArgTypeTest) mkDefault
         else expectedTp match {
-          case ThisType(tref) if tref.symbol.flags is Flags.Module            => and(mkEqualsTest(ref(tref.symbol)), mkTypeTest) // must use == to support e.g. List() == Nil
+          case ThisType(tref) if tref.symbol.flags is Flags.Module            =>
+            and(mkEqualsTest(ref(tref.symbol.companionModule)), mkTypeTest) // must use == to support e.g. List() == Nil
           case ConstantType(Constant(null)) if isAnyRef => mkEqTest(expTp(Literal(Constant(null))))
           case ConstantType(const)                      => mkEqualsTest(expTp(Literal(const)))
           case t:SingletonType                          => mkEqTest(singleton(expectedTp)) // SI-4577, SI-4897